FAILED: patch "[PATCH] mm/shmem: make shmem_file() check the file's inode" failed to apply to 6.1-stable tree

FAILED: patch "[PATCH] mm/shmem: make shmem_file() check the file's inode" failed to apply to 6.1-stable tree

[Sasha Levin]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x d3710111839d11339e414b1bb3fea3d05ab83f6e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109201552-failed-6.1-d3710111839d@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From d3710111839d11339e414b1bb3fea3d05ab83f6e Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Sat, 31 Oct 2026 10:39:32 +0100
Subject: [PATCH] mm/shmem: make shmem_file() check the file's inode

shmem_file() only checks if file->f_mapping is a shmem mapping. This
mapping can belong to another file, however, as with a Coda file backed
by a tmpfs container after mmap().

So it does not guarantee that the file inode is an actual shmem inode.

Seal handling and memfd preservation use this check before accessing
shmem inode state, so they can access an inode of the wrong type.

Let's make shmem_file() check the inode's file operations instead, to
identify shmem inodes reliably. This will identify regular shmem inodes,
including memfds and ordinary tmpfs files, independently of wrapper file
operations.

Use shmem_mapping() in collapse_file() instead to not change the behavior
of collapse_file(): it operates on the page cache and uses mapping->host
for shmem inode operations. Focusing on the file does not seem relevant.

Fixes: c49fcfcda8b5 ("mm/shmem: update file sealing comments and file checking")
Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
Cc: stable@vger.kernel.org
Reported-by: Chris Mason <mason@kernel.org>
Suggested-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Link: https://patch.msgid.link/20261031-fixes_core-v3-3-d6437171ecc3@kernel.org
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 include/linux/shmem_fs.h | 15 ++++++---------
 mm/khugepaged.c          |  2 +-
 mm/shmem.c               |  5 +++++
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index a7c7a96a7cbf9..f89b4b1511332 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -117,8 +117,14 @@ extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);
 #ifdef CONFIG_SHMEM
+bool shmem_file(struct file *file);
 bool shmem_mapping(const struct address_space *mapping);
 #else
+static inline bool shmem_file(struct file *file)
+{
+	return false;
+}
+
 static inline bool shmem_mapping(const struct address_space *mapping)
 {
 	return false;
@@ -193,15 +199,6 @@ static inline struct page *shmem_read_mapping_page(
 					mapping_gfp_mask(mapping));
 }
 
-static inline bool shmem_file(struct file *file)
-{
-	if (!IS_ENABLED(CONFIG_SHMEM))
-		return false;
-	if (!file || !file->f_mapping)
-		return false;
-	return shmem_mapping(file->f_mapping);
-}
-
 /* Must be called with inode lock taken exclusive. */
 static inline void shmem_freeze(struct inode *inode, bool freeze)
 {
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4b71d010b2aff..3a6f4b7b28453 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2230,6 +2230,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 		struct file *file, pgoff_t start, struct collapse_control *cc)
 {
 	struct address_space *mapping = file->f_mapping;
+	const bool is_shmem = shmem_mapping(mapping);
 	struct page *dst;
 	struct folio *folio, *tmp, *new_folio;
 	unsigned long new_pfn = -1;
@@ -2238,7 +2239,6 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
 	enum scan_result result = SCAN_SUCCEED;
 	int nr_none = 0;
-	bool is_shmem = shmem_file(file);
 
 	/*
 	 * MADV_COLLAPSE ignores shmem huge config, so do not check shmem
diff --git a/mm/shmem.c b/mm/shmem.c
index 923957e5e6a24..e64704ac72222 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -276,6 +276,11 @@ static const struct vm_operations_struct shmem_vm_ops;
 static const struct vm_operations_struct shmem_anon_vm_ops;
 static struct file_system_type shmem_fs_type;
 
+bool shmem_file(struct file *file)
+{
+	return file && file_inode(file)->i_fop == &shmem_file_operations;
+}
+
 bool shmem_mapping(const struct address_space *mapping)
 {
 	return mapping->a_ops == &shmem_aops;
-- 
2.53.0